1. Field of the Invention
The present invention relates to a fuel injection system for a reciprocating internal combustion engine, specifically to a fuel injection system for using low viscosity fuel such as alternate fuel to substitute for petroleum fuel.
2. Description of the Related Art
Oil resources are thought to be depleted in the future. To meet the depletion of oil resources, internal combustion engines that use biofuel or reformed natural gas such as DME (dimethyl ether) are disclosed (for example, see Japanese Laid-Open Patent Application No. 2002-309979, No. 2000-120493).
The art disclosed in Japanese Laid-Open Patent Application No. 2002-309979 is to use biogas together with light oil and biofuel is introduced to the suction pipe of the engine. In the art disclosed in Japanese Laid-Open Patent Application No. 2000-120493, liquid dimethyl ether is compressed to high pressure in a fuel injection pump and injected into the combustion chamber of the engine.
There are some kinds of biofuel having a viscosity higher or lower than the usual petroleum fuel. Alternate fuel used to substitute for petroleum, fuel such as DME and GTL, has generally low viscosity in a liquid state. There occurs a problem of insufficient lubrication when compressing low viscosity fuel to high pressure in the fuel injection pump to inject into the combustion chamber of a reciprocating internal engine.
The art disclosed in Japanese Laid-Open Patent Application No. 2002-120493 is the case where an in-line fuel injection pump is used, and proposes a fuel injection system that enables to solve the problem of insufficient lubrication of tappets. This disclosure is applied to the case where fuel injection pressure is not so high and particular consideration is not given to the lubrication of the sliding part of the plunger in a very high pressure injection pump.
When fuel injection pressure is increased in an engine using alternate fuel to substitute for petroleum fuel, it is absolutely necessary to take measures to prevent the occurrence of wear or sticking of the plunger of the fuel injection pump due to insufficient lubrication in the sliding part of the plunger by securing good lubrication, for the alternate fuel has generally low viscosity and poor lubricity.
It is thinkable to add a lubricity improver to fuel in order to secure proper lubrication in the sliding part of the plunger in an engine using low viscosity fuel such as the alternate fuel to substitute for petroleum fuel. However, it is difficult to control so that an additive supplied to a fuel tank is in an appropriate amount for the fuel in the fuel tank.
Further, as the lubricity of fuel in the sliding part of the plunger varies according to the temperature of the fuel in the sliding part, it is necessary to add an additive a little abundantly for safety to secure good lubrication in the sliding part. As emulsion fuel added with water and biofuel contains water, the inside wall of the fuel tank should be specially treated for rust proofing when those kinds of fuel are to be stored as they are in the tank or stored mixed with conventional fuel in the tank.
Further, biofuel is easy to vary in viscosity and other properties with time. Ethanol corrodes metal and rubber, so the fuel tank must be designed to deal with corrosion.
Therefore, in an engine using low viscosity fuel such as alternate fuel to substitute for petroleum fuel, it is desired to solve the problems as described above so that the fuel is supplied with the amount of additive necessary for the fuel in order to secure good lubrication in the sliding part of the plunger. Also, lubrication in the sliding part of the plunger can be improved by mixing biofuel of higher viscosity and conventional fuel with the low viscosity fuel instead of adding an additive for improving lubricity and controlling the viscosity of the mixed fuel so that the mixed fuel has a viscosity similar to that of conventional fuel.
As DME (dimethyl ether) evaporates at a pressure level of 6 kg/cm2 at normal temperatures, it is necessary to provide a means to avoid a decrease in lubricity due to evaporation of DME in the sliding part of the plunger. However, the prior are, including the disclosures in Japanese Laid-Open Patent Applications No. 2002-309979 and No. 2000-120493, does not provide such a means to deal with the problem.